Magnetic field tunable superconducting transition in Nb/Co/Py/Nb exchange spring multilayers

Over the last decade, it has been shown that magnetic non-collinearity at an s-wave superconductor/ferromagnet interface is a key ingredient for spin-singlet to spin-triplet pair conversion. This has been verified in several synthetic non-collinear magnetic structures. A magnetically soft and hard ferromagnetic layer combination in a bilayer structure can function as a field tunable non-collinear magnetic structure, which may offer magnetic field tunability of singlet-to-triplet pair conversion. From magnetization measurements of Nb/Co/Py/Nb multilayers, we demonstrate a reversible enhancement of the superconducting critical temperature of 400 mK by measuring Tc with and without a non-collinear magnetic structure between Co and Py. The sensitivity of Tc in these structures offers the potential for realizing magnetic field tunable Josephson junctions in which pair conversion and Josephson critical currents are controllable using modest magnetic fields.